Mr Skripal and his daughter were poisoned with a nerve agent in Salisbury in March, and have since declined offers of help from their home country of Russia.

But Russian foreign ministry spokeswoman Maria Zakharova says the Skripals are being held in Britain against their will and has claimed the former spy Mr Skripal is still a Russian citizen, despite finding refuge in the UK when he was found to be a double agent.

She said: "We see it as a murder attempt of Russian citizens and huge political provocations and burden of proof lies with UK side. They still refuse to provide consular access saying Yulia does not want to communicate with any Russian representatives while Sergei is a national of UK.

Related: UK Claims Russia Spied On Sergei And Yulia Skripal For At Least 5 Years (Wochit)

"We would like to see proof that they're refusing and we haven't seen that so far," she added.

"We want to hear them say it to us out loud if they don't want any contact.

"We see it as a forceful containment, that they're being taken by UK state - gross violation of bilateral agreements."